U.S. patent number 5,165,464 [Application Number 07/767,260] was granted by the patent office on 1992-11-24 for method of casting hypereutectic aluminum-silicon alloys using a salt core.
This patent grant is currently assigned to Brunswick Corporation. Invention is credited to Terrance M. Cleary, Raymond J. Donahue, William G. Hesterberg, Lawrence I. Toriello.
United States Patent |
5,165,464 |
Donahue , et al. |
November 24, 1992 |
Method of casting hypereutectic aluminum-silicon alloys using a
salt core
Abstract
A method of high pressure casting of hypereutectic
aluminum-silicon alloys using a salt core to form wear resistant
articles, such as engine blocks. To produce an engine block, one or
more solid salt cores are positioned within a metal mold with the
space between the cores and the mold defining a die cavity. A
molten hypereutectic aluminum-silicon alloy containing more than
12% silicon is fed into the die cavity and on solidification of the
molten alloy, precipitated silicon crystals are formed, which are
distributed throughout the wall thickness of the cast part and also
on the surface bordering the salt cores which constitute the
cylinder bores in the cast block. The salt cores are subsequently
removed from the cast block by contact with a solvent such as
water.
Inventors: |
Donahue; Raymond J. (Fond du
Lac, WI), Hesterberg; William G. (Rosendale, WI), Cleary;
Terrance M. (Allenton, WI), Toriello; Lawrence I. (Fond
du Lac, WI) |
Assignee: |
Brunswick Corporation (Skokie,
IL)
|
Family
ID: |
25078961 |
Appl.
No.: |
07/767,260 |
Filed: |
September 27, 1991 |
Current U.S.
Class: |
164/113;
164/522 |
Current CPC
Class: |
B22D
17/24 (20130101); B22D 21/007 (20130101); B22D
29/002 (20130101) |
Current International
Class: |
B22D
21/00 (20060101); B22D 17/24 (20060101); B22D
29/00 (20060101); B22D 017/00 () |
Field of
Search: |
;164/113,132,522 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
4603665 |
August 1986 |
Hesterberg et al. |
4821694 |
April 1989 |
Hesterberg et al. |
4875517 |
October 1989 |
Donahue et al. |
4966220 |
October 1990 |
Hesterberg |
4969428 |
November 1990 |
Donahue et al. |
|
Primary Examiner: Lin; Kuang Y.
Attorney, Agent or Firm: Andrus, Sceales, Starke &
Sawall
Claims
We claim:
1. A method of casting wear resistance components, comprising the
steps of forming an outer metal mold, positioning a salt core in
spaced relation within the mold to provide a die cavity between the
mold and the salt core, introducing a molten hypereutectic
aluminum-silicon alloy into the die cavity at a pressure greater
than 5,000 psi, solidifying the molten alloy to provide a cast
article with precipitated silicon crystals throughout the wall
thickness of the article and at the interface with the salt core,
and thereafter removing the salt core from the cast article.
2. The method of claim I, wherein said alloy contains more than 12%
silicon.
3. The method of claim 1, wherein said salt core is composed of
sodium chloride.
4. The method of claim 1, including the step of feeding the molten
alloy into the die cavity at a pressure of 5,000 to 20,000 psi.
5. The method of claim 1, wherein said alloy has the following
composition in weight percent:
6. A method of casting an engine block for an internal combustion
engine, comprising the steps of forming an outer metal mold,
positioning a plurality of generally cylindrical salt cores within
said mold in spaced relation to said mold and to each other to
provide a die cavity between said cores and said mold, introducing
a molten hypereutectic aluminum-silicon alloy containing more than
12% silicon into said die cavity at a pressure in excess of 5,000
psi solidifying said molten alloy to form a casting with
precipitated silicon particles throughout the wall thickness of
said casting and at the interface with said salt cores, and
thereafter removing said salt cores from said casting to provide a
cast block containing a plurality of cylinder bores.
7. The method of claim 6, wherein said alloy has the following
composition in weight percent:
8. The method of claim 6, wherein the step of removing the salt
cores comprises contacting the cores with a solvent for the
salt.
9. The method of claim 8, wherein the solvent is water.
10. The method of claim 6, wherein said alloy contains by weight
from 12% to 30% silicon, 0.4% to 1.0% magnesium, less than 1.4%
iron, less than 0.3% manganese, less than 0.37% copper, and the
balance aluminum.
Description
BACKGROUND OF THE INVENTION
It has long been recognized that the lighter weight and better heat
transfer properties make aluminum alloys the logical choice as a
material for internal combustion engine blocks. However, most
aluminum alloys lack wear resistance and it has been customary in
the past to chromium plate the cylinder bores in the engine block
with abrasion resistant coatings such as chromium, or alternately,
to apply cast iron liners to the bores. It is difficult to
uniformly plate the cylinder bores and as a result, plating is an
expensive operation, and in the case of chromium plating, not
environmentally friendly. The use of cast iron liners increases the
overall cost of the engine block, as well as the weight of the
engine.
Hypereutectic aluminum-silicon alloys, containing more than I2% by
weight of silicon, possess good wear resistance achieved by the
precipitated silicon crystals which constitute the primary phase.
Because of the wear resistance, attempts have been made to use
hypereutectic aluminum-silicon alloys as casting alloys for engine
blocks to eliminate the need for plated or lined cylinder
bores.
Typical wear resistant aluminum-silicon alloys are described in
U.S. Pat. Nos. 4,603,665 and 4,969,428. U.S. Pat. No. 4,603,665
describes a hypereutectic aluminum-silicon casting alloy having
particular use in casting engine blocks for marine engines. The
alloy of that patent is composed by weight of 16% to 19% silicon,
0.4% to 0.7% magnesium, less than 0.37% copper, and the balance
aluminum. The alloy has a narrow solidification range providing the
alloy with excellent castability, and as the copper content is
maintained at a minimum, the alloy has improved resistance to salt
water corrosion.
U.S. Pat. No. 4,969,428 is directed to a hypereutectic
aluminum-silicon alloy containing in excess of 20% by weight of
silicon and having an improved distribution of primary silicon in
the microstructure. Due to the high silicon content in the alloy,
along with the uniform distribution of the primary silicon in the
microstructure, improved wear resistance is achieved.
High pressure die casting operations have generally been used in
the past to cast engine blocks. In a high pressure die casting
operation, pressures in excess of 5000 psi are normally encountered
and metal molds and cores are employed. Due to the high
metallostatic pressures associated with high pressure die casting,
conventional bonded sand cores are difficult to apply in general
and cannot be employed where size exceeding 10 cubic inches is
involved in conjunction with modest or major geometric shape
complexity, because they do not have the strength to withstand the
high pressures.
It has been found that when using high pressure die casting
operations to cast engine blocks from hypereutectic
aluminum-silicon alloys using a metal mold and metal core, a
denuded zone, free of primary silicon, is formed at the as cast
surface, because of the rapid heat extraction through the metal
core. As the cylindrical surface bordering the metal core
constitutes the cylinder bore in the cast engine block, the denuded
condition adversely effects the wear resistance of the cylinder
bore. Because of this, it has been the practice to remove up to
0.060 inch from the bore by expensive stock removal procedures, and
even with the removal of that thickness, the volume fraction of
primary silicon is often less than that predicted by the phase
diagram. A cylinder bore with a low volume fraction of primary
silicon can cause field failure of the engine, due to the decreased
wear resistance.
SUMMARY OF THE INVENTION
The invention is directed to an improved method of high pressure
casting of hypereutectic aluminum-silicon alloys which will
eliminate denuded zones, free of primary silicon, at the wear
resistant surface. The invention has particular application in
casting engine blocks containing one or more cylinder bores.
In accordance with the invention, a salt core formed of a salt,
such as sodium chloride, is spaced within an outer metal mold, with
the space between the salt core and the mold defining a die cavity.
A molten hypereutectic aluminum silicon alloy containing in excess
of 12% silicon, and preferably in the range of 17% to 30% silicon,
is introduced into the die cavity under high pressure, generally
above 5000 psi. On solidification of the molten alloy, the silicon
will precipitate as silicon crystals which will be distributed
throughout the wall thickness of the cast part, as well as along
the surface bordering the salt core. The salt core, having a low
coefficient of thermal conductivity, will not extract heat from the
molten metal fast enough to suppress the formation of primary
silicon, and as a result there is no detrimental denuded zone
adjacent the salt core that forms the cylinder bore.
With the use of the invention, engine blocks can be produced with
bores that have a uniform distribution of primary silicon at the
original as-cast surface and throughout the wall thickness of the
casting. As there is no denuded zone free of primary silicon,
expensive special stock removal procedures, as used in the past,
are not required.
Further, the use of a salt core results in economic advantages over
the use of metal cores, which must be designed to be split or
collapsible in order to be removed from the cast block.
Other objects and advantages will appear in the course of the
following description.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The invention is directed to a method of high pressure casting of
wear resistant components from a hypereutectic aluminum-silicon
alloy, and has particular application to casting engine blocks for
marine engines. The casting alloy is a hypereutectic
aluminum-silicon alloy containing more than 12% silicon. The alloy
contains precipitated primary silicon particles or crystals that
are distributed throughout the cast part.
In general, the aluminum-silicon alloy contains by weight from 12%
to 30% silicon, 0.4% to 1.0% magnesium, less than 1.45% iron, less
than 0.3% manganese, less than 0.37% copper, and the balance
aluminum.
More particularly the casting alloy can be composed of an
aluminum-silicon alloy as described in U.S. Pat. No. 4,969,428
having the following composition in weight percent:
______________________________________ Silicon 20.0%-30.0%
Magnesium 0.4%-1.6% Iron Less than 1.45% Manganese Less than 0.30%
Copper Less than 0.25% Aluminum Balance
______________________________________
Alternately the casting alloy can be a hypereutectic
aluminum-silicon alloy as described in U.S. Pat. No. 4,821,694
having the following composition in weight percent:
______________________________________ Silicon 16.0-19.0% Magnesium
0.4%-0.7% Iron Less than 1.4% Manganese Less than 0.3% Copper Less
than 0.37% Aluminum Balance
______________________________________
The silicon, being present as discrete precipitated particles or
crystals, contributes to the wear resistance of the alloy.
The magnesium acts to strengthen the alloy through age hardening,
while the iron and manganese tend to harden the alloy, decrease its
ductility, increase its machinability, and aid in maintaining the
mechanical properties of the alloy at elevated temperatures.
By minimizing the copper content, the corrosion resistance of the
alloy to salt water environments is greatly improved.
The alloy can also contain small amounts, up to about 0.2% each, of
residual hardening elements, such as nickel, chromium, zinc or
titanium.
In accordance with the invention, the outer mold used in the high
pressure die casting operation, is formed of a metal, such as
steel, and a salt core is employed. The salt to be used as the core
has a melting point generally above 1200.degree. F and higher than
the melting point of the casting metal, and the salt should be
soluble in a solvent which will not attack the cast metal. For most
applications, a material such as sodium chloride is preferred as
the salt, because it is inexpensive, readily available and can be
solubilized from the metal part by water.
The salt core can be produced by an evaporable foam casting
process, as disclosed in U.S. Pat. No. 4,875,517. As disclosed in
that patent, an evaporable foam pattern is formed of a material
such as polystyrene and has a shape identically proportional to
that of the salt core to be produced. The foam pattern is placed in
a mold and surrounded with an unbonded flowable material, such as
sand. Molten salt, such as sodium chloride, at a temperature
generally in the range of about 1250.degree. F. to 1400.degree. F.,
is then introduced into the mold via a sprue and into contact with
the evaporable foam pattern. The heat of the molten salt vaporizes
the pattern, with the vapor being trapped in the interstices of the
sand, while the molten salt fills the void created by vaporization
of the pattern to produce a salt core identical in configuration to
the evaporable foam pattern.
In the high pressure die casting operation of the invention, the
salt core is positioned in a mold and spaced from the mold to
provide a die cavity. When casting engine blocks, one or more salt
cores are utilized which form the cylinder bores in the cast block.
The cores are spaced inwardly of the metal mold to define the die
cavity.
The molten aluminum-silicon alloy is then introduced into the die
cavity at a high pressure, generally in the range of about 5,000 to
20,000 psi, and preferably about 10,000 psi.
On solidification of the molten metal, primary silicon will be
precipitated and as the salt cores have a relatively low
coefficient of thermal conductivity, generally about 9 Wm.sup.-1.
K.sup.-1, the insulating effect of the cores will prevent rapid
extraction of heat from the molten metal to thereby enable primary
silicon crystals to be precipitated at the interface with the
cores. As a result there is no detrimental denuded zone adjacent
the salt cores that form the engine bores.
Following the casting operation, the cast part is removed from the
die and the salt core is washed from the casting. When using a salt
core formed of a material such as sodium chloride, the core is
preferably removed by immersing the casting in a wash tank
containing water at ambient temperature or at an elevated
temperature. The water is agitated, and depending upon the volume
of the salt core, it will normally be completely dissolved from the
casting in a period of 5 to 30 minutes.
The die cast engine block, or other component, has a uniform
distribution of primary silicon at the original as cast surface
bordering the salt core and throughout the wall thickness of the
casting. As there is no detrimental denuded zone of primary silicon
in the engine bores, expensive special stock removal procedures to
remove metal from the bores is not required.
In addition to preventing the formation of the denuded zones of
primary silicon, the use of the salt cores simplifies the casting
procedure and enables the cores to be formed with more complex or
complicated configurations than when using metal cores.
Various modes of carrying out the invention are contemplated as
being within the scope of the following claims particular1y
pointing out and distinctly claiming the subject matter which is
regarded as the invention.
* * * * *